Random access channel (RACH) of the long term evolution (LTE) system is used for initial network access and uplink timing synchronization. Unlike the legacy 4-step RACH procedure, a 2-step RACH procedure has been discussed in 3GPP standardization meetings for 5G. Note that, compared with the 4-step RACH procedure in the LTE, the simplified 2-step RACH procedure reduces signaling overhead and transmission latency.
Please refer to FIG. 1, which is a schematic diagram of 2-step RACH procedure. In the first step, the UE transmits preamble along with RACH data to the network (i.e. with the message Msg 1). In the second step, the UE receives RACH response including detected preamble index, UE identity, timing advance (TA) from the network (i.e. with the message Msg 2). In other words, the 2-step RACH procedure allows the UE to transmit both preamble and data on the RACH, whereas the 4-step RACH procedure allows the UE to transmit only preamble on the RACH. As a result, the 2-step RACH procedure is beneficial to small-packet uplink transmissions.
However, there is no specification for physical channel design for the 2-step RACH procedure. In detail, demodulation reference signal (DMRS) allocation and numerology/format for RACH data transmission in the message Msg 1 is not considered in the LTE specification. Thus, the network cannot extract/decode the RACH data received from the UE in the 2-step RACH procedure.